Eureka! Mining of Metals in an Ashfill

One successful project in the U.S. State of Maine has been recovering metals from the ashes produced by a waste to energy plant which have been stored in a nearby ashfill for a number of years

While continue to dispose of resource-laden wastes in landfills, we simultaneously seek new sources of virgin materials, especially metals, for use as raw materials. It's a dichotomy that has led many to ask, and with good reason, why don't we mine old landfills?.

By Travis P. Wagner and Kevin Roche

Between 1960 and 2012, over six billion metric tonnes of Municipal Solid Waste (MSW) were disposed of in landfills in the U.S. alone. In addition to paper and plastic, nearly 600,000 tonnes of metals were landfilled during this same period. Using the current market value of loose steel, this equates, conservatively, to more than $114 billion.

However, the primary reason why the mining of landfills for metals has not occurred is that the economic costs are higher than expected revenues. The high costs stem from the technological challenges of separating and recovering metals from the waste. So, when are we going to start mining landfills? We have - at the first-ever successful metals mining operation at a dedicated ashfill in North America.

Buried Resources

Society has long relied on landfilling as the primary means to 'dispose' of waste. Moreover, since 1950, the amount of valuable materials landfilled as MSW also has increased. This is especially evident with the expanded use and thus disposal of higher value electronics, metals, and plastics. As a result, substantial amounts of comparatively concentrated, valuable materials reside in in current and former landfills with access to nearby transportation.

In the U.S. 6270 MSW landfills closed between 1988 and 2005. As of 2012, there were 1908 operating MSW landfills in the U.S. Between 1960 and 2012 approximately. While the concentration of disposed metals in MSW has decreased due to recycling programs, the concentration of metals disposed of in MSW in 2012 was still 8.9%. Between 1960 and 2012, an estimated 590,000 tonnes of metals were landfilled in the U.S.

Given the history of disposing of vast amounts of valuable materials, especially metals, in thousands of landfills, the potential for mining landfills is significant. However, there are many technological and operational barriers, which increase the cost of landfill mining.

Magnetic sorting equipment and eddy current separators are used to recover metals from the ash

Technological Barriers

Landfills can be mined for metals, but, as with any mining operation, the primary decision factor to mine is the economics. Will the economic value of the mined material exceed the cost of mining, segregating, processing, transporting, and restoring the landfill and other concerns unique to landfills?

One of the key characteristics of landfilled MSW is that it is heterogeneous with regards to content, type, and size; it is the heterogeneity coupled with moisture content that cause the technological challenges. The most effective means to recover metals is with magnets for ferrous metals and eddy currents for non-ferrous metals. However, magnets and eddy currents require the waste to be dry and relatively small and uniformed size and the metals need to be unbounded so that the metals can be easily liberated from non-metal portion.

Thus, the more concentrated the metals are, and the more easily the metals are to separate from the waste, the lower the cost. There are, however, additional costs unique to mining landfills. Because of the heterogeneity and lack of information with regards to what and when waste was placed in a landfill, the waste requires extensive analysis, which can be costly. In addition, due to air spaces and active decomposition, landfills are generally less stable and there are additional health and safety concerns due to methane gases and the presence of unknown hazards and contaminated leachate.

Landfilled waste will contain organics (food, paper, wood, etc.), inorganics (metals, plastic, rubber, etc.), inert materials (sand, grit, soil, glass, ceramics, etc.), and liquids, especially water. Metals are often bond within a product. For example, on average, steel makes up 50% of a traditional mattress and box spring set with 27 kg of metal. However, it takes additional steps to liberate the metals and thus is costly to remove and recovery bound metals.

To obtain metals from waste, the waste must be excavated and processed into a smaller, more uniform size to concentrate the metals. When sufficiently processed, the metals can then be separated by magnets for ferrous metals and eddy currents for non-ferrous metals. It is the processing phase that is the challenge because the technologies and the cost to process wet, irregularly shaped waste with bound metals to capture 8% recoverable metal, generally is too high given commodity prices for metals.

Therefore, the commodity price of metals will have to rise significantly, which will happen at some point, or new technology or a rethinking of how to employ existing technology is necessary. It is the latter that holds promise.

The 8 hectare ashfill contains around 725,750 tonnes of ash containing approximately 9% to 16%, by weight of post-burn ferrous metals

A Successful Landfill Mining Operation

In the U.S. state of Maine, the first successful landfill mining operation for metals is occurring. The mining operation is at a dedicated ash landfill (monofill) owned by ecomaine, a non-profit regional waste disposal and recycling operation owned and operated by 21 municipalities. ecomaine, which is located in the southern part of the US state of Maine, handles MSW from approximately 25% of the state's residential population of 1.32 million. ecomaine has operated a mass burn waste to energy (WtE) facility since 1988.

The facility is licensed to process 550 tons (500 tonnes) of MSW per day and generates about 100,000 MWh of electricity per year. ecomaine also owns and operates a 101 hectare landfill located 5 km from the WtE plant and into which an 8 hectare ash monofill is incorporated.

From 1978 until 1988, the landfill accepted raw, baled MSW. Starting in 1988, with the start-up of the WtE plant, only ash has been landfilled. There are two major waste streams that are segregated and collected at the individual generation points - recyclables and waste. The recycling facility was built in 1990 and processed and diverted 5945 kg (Check) of materials in its first year. In 2013, the recycling facility collected and diverted 15,932 kg of materials representing a 168% increase in diversion. Regarding the waste destined for the WtE plant, there is no recovery of metals prior to combustion, rather the metals recovery system follows the combustion process.

The ashfill accepts ash specifically from the WtE facility. Through the burning process, the volume of the waste is reduced to about 90%. Because of the dramatically reduced volume, the metals become concentrated and the ash has a smaller and more uniform size in addition to having most of the moisture removed. Moreover, previously bound metals, such as the steel inside mattresses, are liberated. The dried, concentrated, and smaller particle size ash allows for more efficient and cost-effective sorting and processing to recover metals.

The ecomaine mining operation has focused on recovering post-burn ferrous metal at the portion of the ashfill that has been temporarily closed. The 8 hectare ashfill operated from 1988 to 2009 and received approximately 725,750 tonnes of ash containing 9% to 16%, by weight, of post-burn ferrous metals.

The Feasibility of Recycling

To initiate the project a private company, Reserve Management Group, was contracted to excavate, sample, and analyse ashfill materials to determine the feasibility of recycling ferrous metals from the ash monofill. Based on preliminary estimates of the ferrous metal concentration in the ash, the project was deemed economically viable. Only the ash buried between 1988 and 2004 was targeted for potential mining because 2004 was when the first post-burn magnetic separator was installed at the WtE plant.

The mining operation commenced in November 2011.Excavated ash is transported to the processing area approximately 500 metres from the excavation site. The ash is stockpiled and air dried. If there is precipitation, some ash is washed off the metals and the pile is then left to air dry. Using a front loader, large clumps of ash are broken down, and then loaded onto a conveyor belt where the ash is processed through a series of shaker screens and magnets.

After each screen and magnet configuration, the remaining fraction is transferred to another screen having progressively smaller openings. Material that passes through the screen openings, the under¬size stream, constitutes the soil fraction. Material retained in the screen is removed and is exposed to a magnet to recover ferrous metals. The metals collected include steel cans, nails, automotive parts, springs from mattresses, and many other types of metal materials. The recovered metal is shipped offsite as it requires extensive processing including shredding and grading to separate different grades of material and to remove the ash.

Subsequent analysis of the ash found significant amounts of recoverable non-ferrous metals (i.e., copper, aluminium, stainless steel, and brass) could be recovered. The installation of additional equipment, including eddy current separators, in March 2013 has enabled the reprocessing of ash for the recovery of non-ferrous metals. Thus some of the ash has been processed more than once. While this is not ideal, the reprocessing demonstrates the evolution of learning and the economic benefits of employing off-the-shelf technology. There is also some manual sorting to remove large non-ferrous objects and other metal items that were missed.

Between November 2011 and February this year 24,200 tonnes of metals were recovered and shipped offsite. The estimated total recovery of metals for the entire operation is between 36,500 and 41,000 tonnes. The value of recovered metal thus far is conservatively estimated at $2.32 million.

This is extremely conservative because it assumes all metal is post-burn ferrous, yet significant, but unknown quantities of non-ferrous metals have been recovered, which have a market value of up to 1300% that of ferrous post-burn metal. The total value of metals to be recovered is conservatively estimated at $3.48 to $3.92 million.

In addition the removal of 8028 m3 of material has created valuable new airspace thereby extending the life of the ashfill, which is valued at $430,000 in avoided landfill construction costs.

In addition to the value of the metals recovered for recycling the project is also freeing up valuable void space at the site

Lessons Learned

While this case study has shown that ash monofill mining can be successful using standard off-the-shelf construction equipment such as excavators, front loaders, shaker screens, conveyor belts, and magnets, etc., ash monofills represent a very small percent of landfills.

In the U.S. there are just 32 ashfills, representing only a very small portion of the total number of estimated landfills. The much bigger prize, and bigger challenge, is the mining of with raw (unburned) waste. Raw waste comparatively has a much lower concentration of metals and more processing challenges because of the heterogeneity issues and bounding of metals.

What this case demonstrates, however, is that extracting energy from waste through combustion is a technological means to overcome some of the processing challenges. The emergence of Enhanced Landfill Mining (ELFM) is designed to overcome some of the challenges of mining raw waste as it seeks to extract energy and concentrate the metals to facilitate recovery.

Conclusion

This case study demonstrates that mining of combusted as opposed to raw MSW specifically for the recovery of metals can be successful. Given that metals are a finite resource and landfill capacity is a constant concern, the economics will support landfill mining in the very near future.

With landfill mining, there are other potential economic benefits to the recovery of metals and other resources, which include the value of additional landfill capacity, avoided construction costs, recovered energy values, and avoided remediation costs.

Travis P. Wagner is associate professor of environmental science & policy at the University of Southern Maine's Department of Environmental Science. Kevin Roche is general manager of ecomaine